Add watermelon, sugar, and lemon juice to Instant Pot. Stir thoroughly. Remove any seeds floating on the surface.

Seal the lid, set to high pressure, and cook for 15 minutes. Allow natural pressure release.

Open the lid carefully. Mash the watermelon with a potato masher. Stir in the vanilla extract.

Use the sauté function to simmer the mixture for 10–12 minutes, stirring and mashing occasionally. Skim off any more seeds that rise.

Remove about ¾ cup of liquid from the pot. Mix with cornstarch to form a smooth slurry.

Pour the slurry back into the Instant Pot and stir well. Simmer for another 4–5 minutes until thickened.

Press cancel, remove the pot from the base, and let cool.

Transfer to clean, dry, airtight glass jars. Store in the fridge for up to 2 weeks or freeze for longer.